Photovoltaic device and process for producing same

ABSTRACT

An object of the present invention is to provide a photovoltaic device and a process for producing such a photovoltaic device that enable a stable, high photovoltaic conversion efficiency to be achieved by using a transparent electrode having an optimal relationship between the resistivity and the transmittance. At least one transparent electrode ( 12, 16 ) is either a ZnO layer containing no Ga or a Ga-doped ZnO layer in which the quantity of added Ga is not more than 5 atomic % relative to the Zn within the ZnO layer, and the ZnO layer is formed by a sputtering method using a rare gas containing added oxygen as the sputtering gas, wherein the quantity of oxygen added to the sputtering gas is not less than 0.1% by volume and not more than 5% by volume relative to the combined volume of the oxygen and the rare gas.

TECHNICAL FIELD

The present invention relates to a photovoltaic device having atransparent electrode comprising mainly ZnO (zinc oxide), and a processfor producing the photovoltaic device.

BACKGROUND ART

Conventional photovoltaic devices such as solar cells includesilicon-based thin-film photovoltaic devices. These photovoltaic devicesgenerally comprise a first transparent electrode, silicon-basedsemiconductor layers (photovoltaic layers), a second transparentelectrode, and a metal electrode film laminated sequentially on top of asubstrate.

These transparent electrodes should be made of materials having lowresistance and high light transmittance, and oxide-based transparentconductive films such as ZnO (zinc oxide), SnO₂ (tin oxide), and ITO(indium-tin composite oxide) are used. In order to achieve a lowresistance for this type of transparent electrode, gallium oxide,aluminum oxide, or fluorine or the like is added to the abovetransparent electrode material.

Furthermore, in those cases where thin films of amorphous silicon areused for the photovoltaic layers, a technique in which Ga is added to aZnO layer to enable the transparent electrode film formation to beconducted at low temperatures is also known (for example, see PatentDocument 1 (paragraphs 0006 and 0014, and FIG. 1)).

Patent Document 1:

Japanese Unexamined Patent Application, Publication No. Hei 6-338623

However, the addition of gallium oxide or aluminum oxide to produce alow-resistance transparent electrode results in a decrease in thetransmittance of the transparent electrode. In this manner, addition ofGa or Al to an oxide-based transparent conductive film causes opposingeffects on the resistivity and the transmittance, and achieving acombination of favorable resistivity and favorable transmittance isdifficult.

Furthermore, Patent Document 1 discloses data showing that, in a solarcell that uses amorphous silicon for the photovoltaic layers, theaddition of 0.5 atomic % of Ga relative to Zn in a transparentconductive film comprising mainly ZnO results in increased photovoltaicconversion efficiency compared with the case in which no Ga is added(Example 4 to Example 6 in Table 2), but this technique is merely aninvestigation of the quantity of added Ga required to enable theformation of the transparent conductive film to be conducted at lowertemperatures. In other words, the above technique does not examine thequantity of added Ga required to increase the photovoltaic conversionefficiency by focusing on how the addition of Ga affects either theproperties at the interface between the photovoltaic layer and thetransparent electrode formed from Ga-doped ZnO, or the resistivity andtransmittance of the Ga-doped ZnO layer. Accordingly, a transparentelectrode that is optimized to enable further increases in thephotovoltaic conversion efficiency is still keenly sought.

DISCLOSURE OF INVENTION

The present invention was made in light of the above circumstances, andhas an object of providing a photovoltaic device which, for a range inwhich the properties at the interface between a photovoltaic layer and atransparent electrode comprising Ga-doped Zn are not degraded by theaddition of Ga, achieves a stable, high photovoltaic conversionefficiency by using a transparent electrode having an optimalrelationship between the resistivity and the transmittance, and alsoproviding a process for producing such a photovoltaic device.

In order to achieve the above object, a photovoltaic device of thepresent invention adopts the aspects described below.

Namely, a photovoltaic device according to the present inventioncomprises at least a first transparent electrode, a first photovoltaiclayer containing mainly amorphous silicon or microcrystalline silicon,and a second transparent electrode laminated sequentially on top of anelectrically insulating substrate, wherein at least one of the firsttransparent electrode and the second transparent electrode is either aZnO layer containing no Ga, or a Ga-doped ZnO layer in which thequantity of added Ga is not more than 5 atomic % relative to the Znwithin the ZnO layer, the ZnO layer is formed by a sputtering methodusing a rare gas containing added oxygen as the sputtering gas, and thequantity of oxygen added to the sputtering gas is not less than 0.1% byvolume and not more than 5% by volume relative to the combined volume ofthe oxygen and the rare gas.

Furthermore, a process for producing a photovoltaic device according tothe present invention is a process for producing a photovoltaic devicecomprising at least a first transparent electrode, a first photovoltaiclayer containing mainly amorphous silicon or microcrystalline silicon,and a second transparent electrode laminated sequentially on top of anelectrically insulating substrate, wherein the process comprises a stepof forming at least one of the first transparent electrode and thesecond transparent electrode by a sputtering method that uses a targetcontaining mainly ZnO and a rare gas containing added oxygen as thesputtering gas, the target is either a target containing no Ga, or aGa-doped target in which the quantity of added Ga is not more than 5atomic % relative to the Zn within the ZnO, and the quantity of oxygenadded to the sputtering gas is not less than 0.1% by volume and not morethan 5% by volume relative to the combined volume of the oxygen and therare gas.

The photovoltaic device according to the present invention may be eithera superstrate photovoltaic device in which incident light enters fromthe side of the electrically insulating substrate, or a substratephotovoltaic device in which incident light enters from the oppositeside of the device to the electrically insulating substrate. In the caseof a superstrate photovoltaic device, the above electrically insulatingsubstrate must be a transparent electrically insulating substrate, and aback electrode is formed on the second transparent electrode on theopposite side to the photovoltaic layer. Furthermore, in the case of asubstrate photovoltaic device, the electrically insulating substrate maybe either a non-transparent electrically insulating substrate or atransparent electrically insulating substrate, and the back electrode isformed between this electrically insulating substrate and the firsttransparent electrode.

In the present invention, the first photovoltaic layer contains mainlyamorphous silicon or microcrystalline silicon. The first photovoltaiclayer may have either a PIN structure or a NIP structure, made up of ap-type silicon layer, an i-type silicon layer, and an n-type siliconlayer.

Adding Ga (gallium) oxide to the ZnO (zinc oxide) layer used as atransparent electrode causes the conductivity to increase, but thetransmittance to decrease. As a result of intensive investigation, theinventors of the present invention have discovered that if dueconsideration is given to use of the transparent electrode within aphotovoltaic device, then by maintaining the resistivity at apredetermined level (for example, several Ω·cm) without a great decreasein the transmittance, the photovoltaic conversion efficiency undergoesalmost no reduction. In other words, reducing the quantity of Ga withinthis range in which the conversion efficiency suffers no reduction canbe expected to cause an increase in the conversion efficiency due to anincrease in the transmittance resulting from the decrease in thequantity of Ga. As a result of further investigation based on thisfinding, the inventors discovered that in the case of a singlephotovoltaic device according to the present invention, comprising asingle amorphous silicon layer or single microcrystalline silicon layeras the photovoltaic layer, the photovoltaic conversion efficiency couldbe increased by ensuring that the quantity of added Ga is not more than5 atomic % relative to the quantity of Zn. Moreover, they alsodiscovered that the photovoltaic conversion efficiency could beincreased by forming the Ga-doped ZnO layer by a sputtering method inwhich Ga-doped ZnO is used as the target, and oxygen is added to theargon of the sputtering gas in a quantity of not less than 0.1% byvolume and not more than 5% by volume relative to the combined volume ofthe argon and oxygen within the sputtering gas. The above target may beeither a target containing no Ga, or a Ga-doped target in which thequantity of added Ga is not more than 5 atomic % relative to the Znwithin the ZnO.

In the present invention, a physical vapor deposition method may also beemployed instead of the above sputtering method. In such a case, the ZnOlayer is formed by a physical vapor deposition method using a rare gascontaining added oxygen as the reactive gas, wherein the quantity ofoxygen added to the reactive gas is typically not less than 0.1% byvolume and not more than 5% by volume, and is preferably not less than1% by volume and not more than 3% by volume, relative to the combinedvolume of the oxygen and the rare gas. Furthermore, either a vapordeposition material containing no Ga, or a vapor deposition materialcontaining added Ga in which the quantity of Ga is not more than 5atomic % relative to the Zn within the above ZnO layer can be used.

Because the ZnO layer also has the effect of raising reflectance, theGa-doped ZnO layer is preferably used for the transparent electrodeamongst the first transparent electrode and second transparent electrodethat is positioned adjacent to the back electrode.

As described above, in the present invention, the quantity of added Gais not more than 5 atomic % relative to Zn, and in those cases where theefficiency increases, Ga need not be added (namely, the Ga content maybe 0 atomic %). However, the quantity of added Ga is preferably not lessthan 0.02 atomic % and not more than 2 atomic %, and is even morepreferably not less than 0.7 atomic % and not more than 1.7 atomic %. Inthis description, for the sake of simplicity, ZnO containing not morethan a predetermined quantity of added Ga relative to the Zn is referredto as “Ga-doped Zn”, even in those cases where the ZnO contains no Ga.

The photovoltaic device according to the present invention may also be atandem photovoltaic device in which the aforementioned firstphotovoltaic layer contains mainly microcrystalline silicone, and asecond photovoltaic layer containing mainly amorphous silicon isprovided between this first photovoltaic layer and the aforementionedfirst transparent electrode.

Furthermore, the process for producing a photovoltaic device accordingto the present invention may be a production process in which theaforementioned first photovoltaic layer contains mainly microcrystallinesilicone, wherein the process comprises a step of forming a secondphotovoltaic layer containing mainly amorphous silicon between the firstphotovoltaic layer and the aforementioned first transparent electrode.

In this type of tandem photovoltaic device, in a similar manner to thatdescribed above, adding Ga (gallium) to the ZnO (zinc oxide) layer usedas the transparent electrode causes the conductivity to increase, butthe transmittance to decrease. As a result of intensive investigation,the inventors of the present invention have discovered that if dueconsideration is given to use of the transparent electrode within aphotovoltaic device, then by maintaining the resistivity at apredetermined level (for example, several Ω·cm) without a great decreasein the resistivity, the photovoltaic conversion efficiency undergoesalmost no reduction. In other words, reducing the quantity of Ga withinthis range in which the conversion efficiency suffers no reduction canbe expected to cause an increase in the conversion efficiency due to anincrease in the transmittance resulting from the decrease in thequantity of Ga. Moreover, this increase in the conversion efficiency isenhanced by adding oxygen to the atmosphere during sputtering. As aresult of further investigation based on this finding, the inventorsdiscovered that in the case of a tandem photovoltaic device according tothe present invention, comprising a microcrystalline silicon layer (afirst photovoltaic layer) and an amorphous silicon layer (a secondphotovoltaic layer) as the two photovoltaic layers, the photovoltaicconversion efficiency could be increased by ensuring that the quantityof added Ga is not more than 5 atomic % relative to the quantity of Zn.Moreover, they also discovered that the photovoltaic conversionefficiency could be increased by forming the Ga-doped ZnO layer by asputtering method in which Ga-doped ZnO is used as the target, andoxygen is added to the argon of the sputtering gas in a quantity of notless than 0.1% by volume and not more than 5% by volume relative to thecombined volume of argon and oxygen within the sputtering gas.

In the above tandem photovoltaic device according to the presentinvention, in a similar manner to that described above, a physical vapordeposition method may be employed instead of the above sputteringmethod. In such a case, the ZnO layer is formed by a physical vapordeposition method using a rare gas containing added oxygen as thereactive gas, wherein the quantity of oxygen added to the reactive gasis typically not less than 0.1% by volume and not more than 5% byvolume, and is preferably not less than 1% by volume and not more than3% by volume, relative to the combined volume of the oxygen and the raregas.

Because the ZnO layer also has the effect of raising reflectance, in thetandem photovoltaic device according to the present invention, aGa-doped ZnO layer is preferably used for the second transparentelectrode positioned adjacent to the back electrode, or for the firsttransparent electrode positioned adjacent to a non-transparentelectrically insulating substrate.

In the present invention described above, the rare gas used withineither the sputtering gas used in the sputtering method or the reactivegas used in the physical vapor deposition method can use argon, neon,krypton or xenon or the like, although the use of argon is particularlyfavorable.

According to the present invention, because the quantity of added Ga isreduced as far as possible within a range that enables the desiredphotovoltaic conversion efficiency to be maintained, with some allowancefor an increase in the resistivity of the transparent electrode,reductions in the transmittance caused by the Ga addition can besuppressed, enabling the production of a transparent electrode with hightransmittance over a wide range of wavelengths.

Because a high transmittance is achieved in this manner, more intenselight can be supplied to the photovoltaic layers, thereby increasing theshort-circuit current density, and as a result, increasing thephotovoltaic conversion efficiency.

Furthermore, suppressing the quantity of added Ga enhances theproperties at the interface with the n-type silicon layer, enabling ahigh open-circuit voltage, a favorable short-circuit current density anda favorable fill factor to be achieved, and as a result, thephotovoltaic conversion efficiency improves.

Furthermore, according to the present invention, by adding apredetermined quantity of oxygen during the formation of the transparentelectrode that contains Ga-doped ZnO, either to the sputtering gas usedin the sputtering method, or to the reactive gas used in the physicaldeposition method, the partial pressure of water vapor inside thetransparent electrode film formation device, which has an adverse effecton ZnO oxidation, is reduced by a comparative amount, and as a result,the transmittance of the transparent electrode containing the Ga-dopedZnO is stabilized, meaning the photovoltaic conversion efficiency of thefinal photovoltaic device is also stabilized.

BRIEF DESCRIPTION OF DRAWINGS

[FIG. 1] A cross-sectional view showing a schematic representation of asingle photovoltaic device according to a first embodiment of thepresent invention, which includes an amorphous silicon photovoltaiclayer in which incident light enters from the side of a transparentelectrically insulating substrate.

[FIG. 2] A cross-sectional view showing a schematic representation of asingle photovoltaic device according to a second embodiment of thepresent invention, which includes a microcrystalline siliconphotovoltaic layer in which incident light enters from the side of atransparent electrically insulating substrate.

[FIG. 3] A cross-sectional view showing a schematic representation of atandem photovoltaic device according to a third embodiment of thepresent invention, which includes an amorphous silicon photovoltaiclayer and a microcrystalline silicon photovoltaic layer, and in whichincident light enters from the side of a transparent electricallyinsulating substrate.

[FIG. 4] A graph showing the conversion efficiency of photovoltaicdevices relative to the resistivity of the transparent electrode.

[FIG. 5] A graph showing the resistivity for Ga-doped ZnO layers formedusing a sputtering method, for different quantities of added Ga anddifferent quantities of oxygen within the sputtering gas.

EXPLANATION OF REFERENCE SIGNS

-   11: Transparent electrically insulating substrate-   17: Back electrode-   12,22,32: First transparent electrode-   16,26,46: Second transparent electrode-   10,30: Amorphous silicon photovoltaic layer-   20,40: Microcrystalline silicon photovoltaic layer

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments according to the present invention are described below withreference to the drawings.

First Embodiment

A photovoltaic device according to a first embodiment of the presentinvention is described below with reference to FIG. 1.

The photovoltaic device according to this embodiment has a photovoltaiclayer 10 of amorphous silicon, and incident light enters from thetransparent electrically insulating substrate (also referred to as asuperstrate device).

(First Step)

A first transparent electrode 12 is formed on a transparent electricallyinsulating substrate 11. Optically transparent white crown glass, forexample, can be used for the transparent electrically insulatingsubstrate 11.

The first transparent electrode 12 is formed using SnO₂ (tin oxide).

The transparent electrically insulating substrate 11 is housed inside anormal pressure heated CVD apparatus, and a film of SnO₂ is formed onthe transparent electrically insulating substrate 11 using SnCl₄, watervapor (H₂O) and anhydrous hydrogen fluoride (HF) as the raw materialgases.

(Second Step)

Subsequently, with the transparent electrically insulating substrate 11on which the first transparent electrode 12 has been formed held as aprocessing object at the anode of a plasma enhanced CVD apparatus, theprocessing object is housed in a reaction chamber, and a vacuum pump isthen activated and used to evacuate the interior of the reaction chamberto a vacuum. Subsequently, electricity is supplied to a heaterincorporated within the anode, and the substrate of the processingobject is heated, for example to 160° C. or higher. SiH₄, H₂, and ap-type dopant gas, which function as the raw material gases, are thenintroduced into the reaction chamber, and the pressure inside thereaction chamber is regulated at a predetermined level. A plasma is thengenerated between a discharge electrode and the processing object bysupplying RF electrical power from an RF power supply to the dischargeelectrode, thereby forming an amorphous p-type silicon layer 13 on thefirst transparent electrode 12 of the processing object.

B₂H₆ or the like can be used as the p-type dopant gas.

(Third Step)

Once the p-type silicon layer 13 has been formed, the transparentelectrically insulating substrate 11 is housed inside the reactionchamber of another plasma enhanced CVD apparatus, and the interior ofthe reaction chamber is evacuated to a vacuum. A mixed gas of SiH₄ andH₂ that functions as the raw material gas is then introduced into thereaction chamber, and the pressure inside the reaction chamber isregulated at a predetermined level. A plasma is then generated between adischarge electrode and the processing object by supplying very highfrequency electrical power with a frequency of 60 MHz or higher from avery high frequency power supply to the discharge electrode, therebyforming an amorphous i-type silicon layer 14 on the p-type silicon layer13 of the processing object.

Furthermore, the pressure during generation of the plasma inside thereaction chamber is preferably set to a value within a range from notless than 0.5 Torr to not more than 10 Torr, and even more preferably toa value within a range from not less than 0.5 Torr to not more than 6.0Torr.

(Fourth Step)

Once the i-type silicon layer 14 has been formed, supply of the rawmaterial gas is halted, and the interior of the reaction chamber isevacuated to a vacuum. Subsequently, the transparent electricallyinsulating substrate 11 is housed inside another reaction chamber thathas been evacuated to a vacuum, and SiH₄, H₂, and an n-type dopant gas(such as PH₃), which function as the raw material gases, are introducedinto this reaction chamber, and the pressure inside the reaction chamberis regulated at a predetermined level. A plasma is then generatedbetween a discharge electrode and the processing object by supplyingvery high frequency electrical power from a very high frequency powersupply to the discharge electrode, thereby forming an amorphous n-typesilicon layer 15 on the i-type silicon layer 14. The processing objectis then removed from the plasma enhanced CVD apparatus.

In this manner, by executing the second to fourth steps, an amorphoussilicon photovoltaic layer 10 comprising the p-type silicon layer 13,the i-type silicon layer 14, and the n-type silicon layer 15 is formed.

(Fifth Step)

Once the n-type silicon layer 15 has been formed, supply of the rawmaterial gases is halted, and the interior of the reaction chamber isevacuated to a vacuum. Subsequently, the transparent electricallyinsulating substrate 11 with the layers up to and including the n-typesilicon layer 15 formed thereon is housed inside a direct currentsputtering (DC sputtering) apparatus.

In this DC sputtering apparatus, a Ga-doped ZnO layer is formed as asecond transparent electrode 16 on the n-type silicon layer 15.

Once the transparent electrically insulating substrate 11 has beenhoused inside the DC sputtering apparatus, DC sputtering is conductedwithin an evacuated atmosphere into which a predetermined quantity of amixed gas of argon gas and oxygen gas has been introduced, therebyforming the Ga-doped ZnO layer on the n-type silicon layer 15. Thequantity of added Ga relative to Zn is not more than 5 atomic %, ispreferably not less than 0.02 atomic % and not more than 2 atomic %, andis even more preferably not less than 0.7 atomic % and not more than 1.7atomic %. Furthermore, the quantity of added oxygen relative to thecombined volume of argon and oxygen within the sputtering gas is set toa value of not less than 0.1% by volume and not more than 5% by volume.

The pressure inside the DC sputtering apparatus is preferablyapproximately 0.6 Pa, the temperature of the transparent electricallyinsulating substrate 11 is preferably not less than 80° C. and not morethan 135° C., and the sputtering power is preferably approximately 100W.

Adding Ga to a transparent electrode formed from ZnO causes theconductivity to increase, but the transmittance to decrease. As a resultof intensive investigation, the inventors of the present inventiondiscovered that if due consideration is given to use of the transparentelectrode within a photovoltaic device, then by maintaining theresistivity at a predetermined level (for example, several Ω·cm) withouta great decrease in the resistivity, the photovoltaic conversionefficiency increases.

FIG. 4 shows the relationship between the resistivity of a transparentelectrode formed from Ga-doped ZnO (horizontal axis) and the conversionefficiency of a photovoltaic device (vertical axis). In the graph ofFIG. 4, the top two lines represent data for tandem photovoltaic deviceswith different thickness values for the i-layer, the third line from thetop represents data for a single photovoltaic device having an amorphoussilicon photovoltaic layer, and the bottom two lines represent data forsingle photovoltaic devices having a microcrystalline siliconphotovoltaic layer with different thickness values for the i-layer. FromFIG. 4 it is evident that even if the resistivity of the transparentelectrode is raised to approximately 50 Ω·cm, the photovoltaicconversion efficiency does not decrease. Accordingly, if the quantity ofadded Ga is reduced within this range for which the photovoltaicconversion efficiency does not decrease, then the conversion efficiencycan be expected to increase due to an increase in the transmittanceresulting from the reduction in the quantity of added Ga. Furthermore,by adding oxygen to the argon of the sputtering gas, the transmittancecan be increased even further. Moreover, the reduction in the quantityof Ga also improves the properties at the interface between the n-layerand the Ga-doped ZnO.

In this embodiment, as a result of further investigation on the quantityof added Ga based on the above findings, the quantity of Ga addedrelative to the Zn with the second transparent electrode 16 isrestricted to not more than 5 atomic % in the case of a singlephotovoltaic device comprising a single amorphous silicon photovoltaiclayer 10 according to the present embodiment. Furthermore, oxygen isadded to the sputtering gas in a quantity of not less than 0.1% byvolume and not more than 5% by volume relative to the combined volume ofargon and oxygen within the sputtering gas. The inventors discoveredthat provided the above conditions were satisfied, the photovoltaicconversion efficiency could be increased.

FIG. 5 is a graph showing the resistivity for a transparent electrodeunder a variety of conditions in which the quantity of oxygen relativeto the combined volume of argon and oxygen within the sputtering gas isvaried between 0.1% by volume, 1% by volume, 2% by volume and 5% byvolume, and the quantity of added Ga relative to Zn within the Ga-dopedZnO transparent electrode is varied within the range specified by thepresent invention.

In the case where a physical vapor deposition method is conductedinstead of the sputtering method, a rare gas containing added oxygen isused as the reactive gas, and the quantity of oxygen added is not lessthan 0.1% by volume and not more than 5% by volume relative to thecombined volume of argon and oxygen within the reactive gas.

(Sixth Step)

Subsequently, an Ag film or Al film is formed as a back electrode 17 onthe second transparent electrode 16. A photovoltaic device produced inthis manner generates electricity by photovoltaic conversion fromincident light such as sunlight that enters the amorphous silicon layerwith the PIN structure described above via the transparent electricallyinsulating substrate 11.

In the production of the photovoltaic device, the photovoltaic layer 10was formed with a PIN structure by sequential formation of the p-typesilicon layer 13, the i-type silicon layer 14, and the n-type siliconlayer 15 on top of the first transparent electrode 12, but thephotovoltaic layer 10 may also be formed with a NIP structure bysequential formation of an n-type silicon layer, i-type silicon layer,and p-type silicon layer.

Furthermore, in this embodiment, the ZnO layer in which the quantity ofadded Ga relative to Zn was restricted to not more than 5 atomic % andthe quantity of oxygen added was restricted to not more than 5% byvolume was used for the second transparent electrode 16, but the presentinvention is not limited to this case, and the above ZnO layer may alsobe used for the first transparent electrode 12.

However, because the transparent electrode also has the effect ofincreasing the reflectance, the Ga-doped ZnO layer of the presentinvention is preferably employed as the second transparent electrode 16positioned adjacent to the back electrode 17.

According to this embodiment, Ga-doped ZnO was employed as the secondtransparent electrode 16, the quantity of added Ga relative to Zn wasrestricted to not more than 5 atomic %, the quantity of oxygen added tothe sputtering gas during formation of the Ga-doped ZnO was set to avalue of not less than 0.1% by volume and not more than 5% by volumerelative to the combined volume of argon and oxygen within thesputtering gas, and the quantity of added Ga was reduced as far aspossible within a range that enabled the desired photovoltaic conversionefficiency to be maintained, with some allowance for an increase in theresistivity of the second transparent electrode 16, and as a result,reductions in the transmittance were suppressed, enabling the productionof a second transparent electrode 16 with high transmittance over a widerange of wavelengths. Furthermore, by adding oxygen to the filmformation atmosphere, a more stable production that is unaffected byoutgas from the vacuum chamber can be achieved.

Because a high transmittance is achieved in this manner, more intenselight can be supplied to the photovoltaic layer 10, thereby increasingthe short-circuit current density, and as a result, increasing thephotovoltaic conversion efficiency.

Second Embodiment

A photovoltaic device according to a second embodiment of the presentinvention is described below with reference to FIG. 2.

The photovoltaic device according to this embodiment has a photovoltaiclayer 20 of microcrystalline silicon, and incident light enters from thetransparent electrically insulating substrate. Although the photovoltaicdevice according to this embodiment has an electricity-generating layermade of microcrystalline silicon, the incident light enters from thetransparent electrically insulating substrate in the same manner as thefirst embodiment (namely, a superstrate device).

(First Step)

A first transparent electrode 22 is formed on a transparent electricallyinsulating substrate 11. Optically transparent white crown glass, forexample, can be used for the transparent electrically insulatingsubstrate 11.

The first transparent electrode 22 is formed using SnO₂ (tin oxide).

The transparent electrically insulating substrate 11 is housed inside anormal pressure heated CVD apparatus, and a film of SnO₂ is formed onthe transparent electrically insulating substrate 11 using SnCl₄, watervapor (H₂O) and anhydrous hydrogen fluoride (HF) as the raw materialgases.

(Second Step)

Subsequently, with the transparent electrically insulating substrate 11on which the first transparent electrode 22 has been formed held as aprocessing object at the anode of a plasma enhanced CVD apparatus, theprocessing object is housed in a reaction chamber, and a vacuum pump isthen activated and used to evacuate the interior of the reaction chamberto a vacuum. Subsequently, electricity is supplied to a heaterincorporated within the anode, and the substrate of the processingobject is heated, for example to 160° C. or higher. SiH₄, H₂, and ap-type dopant gas, which function as the raw material gases, are thenintroduced into the reaction chamber, and the pressure inside thereaction chamber is regulated at a predetermined level. A plasma is thengenerated between a discharge electrode and the processing object bysupplying very high frequency electrical power from a very highfrequency power supply to the discharge electrode, thereby forming amicrocrystalline p-type silicon layer 23 on the first transparentelectrode 22 of the processing object.

B₂H₆ or the like can be used as the p-type dopant gas.

(Third Step)

Once the p-type silicon layer 23 has been formed, the transparentelectrically insulating substrate 11 is housed inside the reactionchamber of another plasma enhanced CVD apparatus, and the interior ofthe reaction chamber is evacuated to a vacuum. A mixed gas of SiH₄ andH₂ that functions as the raw material gas is then introduced into thereaction chamber, and the pressure inside the reaction chamber isregulated at a predetermined level. A plasma is then generated between adischarge electrode and the processing object by supplying very highfrequency electrical power with a frequency of 60 MHz or higher from avery high frequency power supply to the discharge electrode, therebyforming a microcrystalline i-type silicon layer 24 on the p-type siliconlayer 23 of the processing object.

Furthermore, the pressure during generation of the plasma inside thereaction chamber is preferably set to a value within a range from notless than 0.5 Torr to not more than 10 Torr, and even more preferably toa value within a range from not less than 1.0 Torr to not more than 6.0Torr.

(Fourth Step)

Once the i-type silicon layer 24 has been formed, supply of the rawmaterial gas is halted, and the interior of the reaction chamber isevacuated to a vacuum. Subsequently, the transparent electricallyinsulating substrate 11 is housed inside another reaction chamber thathas been evacuated to a vacuum, and SiH₄, H₂, and an n-type dopant gas(such as PH₃), which function as the raw material gases, are introducedinto this reaction chamber, and the pressure inside the reaction chamberis regulated at a predetermined level. A plasma is then generatedbetween a discharge electrode and the processing object by supplyingvery high frequency electrical power from a very high frequency powersupply to the discharge electrode, thereby forming a microcrystallinen-type silicon layer 25 on the i-type silicon layer 24. The processingobject is then removed from the plasma enhanced CVD apparatus. In thismanner, by executing the second to fourth steps, a microcrystallinesilicon photovoltaic layer 20 comprising the p-type silicon layer 23,the i-type silicon layer 24, and the n-type silicon layer 25 is formed.

(Fifth Step)

Once the n-type silicon layer 25 has been formed, supply of the rawmaterial gases is halted, and the interior of the reaction chamber isevacuated to a vacuum. Subsequently, the transparent electricallyinsulating substrate 11 with the layers up to and including the n-typesilicon layer 25 formed thereon is housed inside a DC sputteringapparatus.

In this DC sputtering apparatus, a Ga-doped ZnO layer is formed as asecond transparent electrode 26 on the n-type silicon layer 25.

Once the transparent electrically insulating substrate 11 has beenhoused inside the DC sputtering apparatus, DC sputtering is conductedwithin an evacuated atmosphere into which a predetermined quantity ofargon gas has been introduced, thereby forming the Ga-doped ZnO layer onthe n-type silicon layer 25. The quantity of added Ga relative to Zn isnot more than 5 atomic %, is preferably not less than 0.02 atomic % andnot more than 2 atomic %, and is even more preferably not less than 0.7atomic % and not more than 1.7 atomic %. Furthermore, the quantity ofadded oxygen relative to the combined volume of argon gas and oxygenwithin the sputtering gas is set to a value of not less than 0.1% byvolume and not more than 5% by volume.

The pressure inside the DC sputtering apparatus is preferablyapproximately 0.6 Pa, the temperature of the transparent electricallyinsulating substrate 11 is preferably not less than 80° C. and not morethan 135° C., and the sputtering power is preferably approximately 100W.

The reasons that the quantity of added Ga and the quantity of addedoxygen were selected in the manner described above are as describedabove for the first embodiment with reference to FIG. 4. Namely, if thequantity of added Ga is reduced within the range for which theconversion efficiency for the photovoltaic device does not decrease,then the conversion efficiency can be expected to increase due to anincrease in the transmittance resulting from the reduction in thequantity of added Ga, and furthermore, if the quantity of added oxygenis increased, then the conversion efficiency can be expected to increasedue to an increase in the transmittance. Moreover, the reduction in thequantity of Ga also improves the properties at the interface between then-layer and the Ga-doped ZnO. In this embodiment, as a result ofinvestigations from the above perspectives of the quantity of added Gaand the quantity of added oxygen, the quantity of Ga added relative toZn in the second transparent electrode 26 is restricted to not more than5 atomic % for the case of a single photovoltaic device comprising asingle microcrystalline silicon photovoltaic layer 20 according to thepresent invention. Furthermore, oxygen is added to the sputtering gas insufficient quantity that the volume of oxygen relative to the combinedvolume of argon and oxygen within the sputtering gas is not less than0.1% by volume and not more than 5% by volume. It was discovered thatprovided these conditions were satisfied, the photovoltaic conversionefficiency could be increased.

(Sixth Step)

Subsequently, an Ag film or Al film is formed as a back electrode 27 onthe second transparent electrode 26 using a sputtering method or vacuumvapor deposition method.

A photovoltaic device produced in this manner generates electricity byphotovoltaic conversion from incident light such as sunlight that entersthe microcrystalline silicon layer with the PIN structure describedabove via the transparent electrically insulating substrate 11.

In the production of the photovoltaic device, the photovoltaic layer 20was formed with a PIN structure by sequential formation of the p-typesilicon layer 23, the i-type silicon layer 24, and the n-type siliconlayer 25 on top of the first transparent electrode 22, but thephotovoltaic layer 20 may also be formed with a NIP structure bysequential formation of an n-type silicon layer i-type silicon layer,and p-type silicon layer.

Furthermore, in this embodiment, the ZnO layer in which the quantity ofadded Ga relative to Zn was restricted to not more than 5 atomic % wasused for the second transparent electrode 26, but the present inventionis not limited to this case, and the above ZnO layer may also be usedfor the first transparent electrode 22.

However, because the transparent electrode also has the effect ofincreasing the reflectance, the Ga-doped ZnO layer of the presentinvention is preferably employed as the second transparent electrode 26positioned adjacent to the back electrode 27.

According to this embodiment, Ga-doped ZnO was employed as the secondtransparent electrode 26, the quantity of added Ga relative to Zn wasrestricted to not more than 5 atomic %, the quantity of oxygen added tothe sputtering gas during formation of the Ga-doped ZnO layer was set toa value of not less than 0.1% by volume and not more than 5% by volumerelative to the combined volume of argon and oxygen within thesputtering gas, and the quantity of added Ga was reduced as far aspossible within a range that enabled the desired photovoltaic conversionefficiency to be maintained, with some allowance for an increase in theresistivity of the second transparent electrode 26, and as a result,reductions in the transmittance were suppressed, enabling the productionof a second transparent electrode 26 with high transmittance over a widerange of wavelengths. Furthermore, by adding oxygen to the filmformation atmosphere, a more stable production that is unaffected byoutgas from the vacuum chamber can be achieved.

Because a high transmittance is achieved in this manner, more intenselight can be supplied to the photovoltaic layer 20, thereby increasingthe short-circuit current density, and as a result, increasing thephotovoltaic conversion efficiency.

Furthermore, suppressing the quantity of added Ga enhances the interfaceproperties with the p-type and n-type silicon layers, enabling a highopen-circuit voltage, a favorable short-circuit current density and afavorable fill factor to be achieved, and as a result, the photovoltaicconversion efficiency improves.

Third Embodiment

A photovoltaic device according to a third embodiment of the presentinvention is described below with reference to FIG. 3.

The photovoltaic device according to this embodiment differs from eachof the above embodiments in that it is a tandem device in which thephotovoltaic layer comprises an amorphous silicon photovoltaic layer 30(a second photovoltaic layer) and a microcrystalline siliconphotovoltaic layer 40 (a first photovoltaic layer) laminated together.The photovoltaic device according to this embodiment is similar to thefirst embodiment and second embodiment in that the incident light entersfrom the transparent electrically insulating substrate (namely, asuperstrate device).

(First Step)

A first transparent electrode 32 is formed on a transparent electricallyinsulating substrate 11. Optically transparent white crown glass, forexample, can be used for the transparent electrically insulatingsubstrate 11.

The first transparent electrode 32 is formed using SnO₂ (tin oxide).

The transparent electrically insulating substrate 11 is housed inside anormal pressure heated CVD apparatus, and a film of SnO₂ is formed onthe transparent electrically insulating substrate 11 using SnCl₄, watervapor (H₂O) and anhydrous hydrogen fluoride (HF) as the raw materialgases.

(Second Step)

Subsequently, with the transparent electrically insulating substrate 11on which the first transparent electrode 32 has been formed held as aprocessing object at the anode of a plasma enhanced CVD apparatus, theprocessing object is housed in a reaction chamber, and a vacuum pump isthen activated and used to evacuate the interior of the reaction chamberto a vacuum. Subsequently, electricity is supplied to a heaterincorporated within the anode, and the substrate of the processingobject is heated, for example to 160° C. or higher. SiH₄, H₂, and ap-type dopant gas, which function as the raw material gases, are thenintroduced into the reaction chamber, and the pressure inside thereaction chamber is regulated at a predetermined level. A plasma is thengenerated between a discharge electrode and the processing object bysupplying RF electrical power from an RF power supply to the dischargeelectrode, thereby forming an amorphous p-type silicon layer 33 on thefirst transparent electrode 32 of the processing object.

B₂H₆ or the like can be used as the p-type dopant gas.

(Third Step)

Once the p-type silicon layer 33 has been formed, the transparentelectrically insulating substrate 11 is housed inside the reactionchamber of another plasma enhanced CVD apparatus, and the interior ofthe reaction chamber is evacuated to a vacuum. A mixed gas of SiH₄ andH₂ that functions as the raw material gas is then introduced into thereaction chamber, and the pressure inside the reaction chamber isregulated at a predetermined level. A plasma is then generated between adischarge electrode and the processing object by supplying very highfrequency electrical power with a frequency of 60 MHz or higher from avery high frequency power supply to the discharge electrode, therebyforming an amorphous i-type silicon layer 34 on the p-type silicon layer33 of the processing object.

Furthermore, the pressure during generation of the plasma inside thereaction chamber is preferably set to a value within a range from notless than 0.5 Torr to not more than 10 Torr, and even more preferably toa value within a range from not less than 0.5 Torr to not more than 6.0Torr.

(Fourth Step)

Once the i-type silicon layer 34 has been formed, supply of the rawmaterial gas is halted, and the interior of the reaction chamber isevacuated to a vacuum. Subsequently, the transparent electricallyinsulating substrate 11 is housed inside another reaction chamber thathas been evacuated to a vacuum, and SiH₄, H₂, and an n-type dopant gas(such as PH₃), which function as the raw material gases, are introducedinto this reaction chamber, and the pressure inside the reaction chamberis regulated at a predetermined level. A plasma is then generatedbetween a discharge electrode and the processing object by supplyingvery high frequency electrical power from a very high frequency powersupply to the discharge electrode, thereby forming an amorphous n-typesilicon layer 35 on the i-type silicon layer 34. The processing objectis then removed from the plasma enhanced CVD apparatus.

In this manner, by executing the second to fourth steps, an amorphoussilicon photovoltaic layer 30 comprising the p-type silicon layer 33,the i-type silicon layer 34, and the n-type silicon layer 35 is formed.

(Fifth Step)

Next, a microcrystalline silicon photovoltaic layer 40 is formed on topof the above amorphous silicon photovoltaic layer 30.

The method of forming the microcrystalline silicon photovoltaic layer 40is the same as that described for the second embodiment.

In other words, with the transparent electrically insulating substrate11 on which the amorphous silicon photovoltaic layer 30 has been formedheld as a processing object at the anode of a plasma enhanced CVDapparatus, the processing object is housed in a reaction chamber, and avacuum pump is then activated and used to evacuate the interior of thereaction chamber to a vacuum. Subsequently, electricity is supplied to aheater incorporated within the anode, and the substrate of theprocessing object is heated, for example to 160° C. or higher. SiH₄, H₂,and a p-type dopant gas, which function as the raw material gases, arethen introduced into the reaction chamber, and the pressure inside thereaction chamber is regulated at a predetermined level. A plasma is thengenerated between a discharge electrode and the processing object bysupplying very high frequency electrical power from a very highfrequency power supply to the discharge electrode, thereby forming amicrocrystalline p-type silicon layer 43 on the amorphous siliconphotovoltaic layer 30 of the processing object.

B₂H₆ or the like can be used as the p-type dopant gas.

(Sixth Step)

Once the p-type silicon layer 43 has been formed, the transparentelectrically insulating substrate 11 is housed inside the reactionchamber of another plasma enhanced CVD apparatus, and the interior ofthe reaction chamber is evacuated to a vacuum. A mixed gas of SiH₄ andH₂ that functions as the raw material gas is then introduced into thereaction chamber, and the pressure inside the reaction chamber isregulated at a predetermined level. A plasma is then generated between adischarge electrode and the processing object by supplying very highfrequency electrical power with a frequency of 60 MHz or higher from avery high frequency power supply to the discharge electrode, therebyforming a microcrystalline i-type silicon layer 44 on the p-type siliconlayer 43 of the processing object.

Furthermore, the pressure during generation of the plasma inside thereaction chamber is preferably set to a value within a range from notless than 0.5 Torr to not more than 10 Torr, and even more preferably toa value within a range from not less than 1.0 Torr to not more than 6.0Torr.

(Seventh Step)

Once the i-type silicon layer 44 has been formed, supply of the rawmaterial gas is halted, and the interior of the reaction chamber isevacuated to a vacuum. Subsequently, the transparent electricallyinsulating substrate 11 is housed inside another reaction chamber thathas been evacuated to a vacuum, and SiH₄, H₂, and an n-type dopant gas(such as PH₃), which function as the raw material gases, are introducedinto this reaction chamber, and the pressure inside the reaction chamberis regulated at a predetermined level. A plasma is then generatedbetween a discharge electrode and the processing object by supplyingvery high frequency electrical power from a very high frequency powersupply to the discharge electrode, thereby forming a microcrystallinen-type silicon layer 45 on the i-type silicon layer 44. The processingobject is then removed from the plasma enhanced CVD apparatus.

In this manner, by executing the fifth to seventh steps, amicrocrystalline silicon photovoltaic layer 40 comprising the p-typesilicon layer 43, the i-type silicon layer 44, and the n-type siliconlayer 45 is formed.

(Eighth Step)

Once the n-type silicon layer 45 has been formed, supply of the rawmaterial gases is halted, and the interior of the reaction chamber isevacuated to a vacuum. Subsequently, the transparent electricallyinsulating substrate 11 with the layers up to and including the n-typesilicon layer 45 formed thereon is housed inside a DC sputteringapparatus.

In this DC sputtering apparatus, a Ga-doped ZnO layer is formed as asecond transparent electrode 46 on the n-type silicon layer 45.

Once the transparent electrically insulating substrate 11 has beenhoused inside the DC sputtering apparatus, DC sputtering is conductedwithin an evacuated atmosphere into which a predetermined quantity ofargon gas has been introduced, thereby forming the Ga-doped ZnO layer onthe n-type silicon layer 45. The quantity of added Ga relative to Zn isnot more than 5 atomic %, is preferably not less than 0.02 atomic % andnot more than 2 atomic %, and is even more preferably not less than 0.7atomic % and not more than 1.7 atomic %. The reasons that the quantityof added Ga is selected from within this numerical range are the same asthose described above for the first embodiment, and consequently, anexplanation of those reasons is omitted here.

The pressure inside the DC sputtering apparatus is preferablyapproximately 0.6 Pa, the temperature of the transparent electricallyinsulating substrate 11 is preferably not less than 80° C. and not morethan 135° C., and the sputtering power is preferably approximately 100W.

(Ninth Step)

Subsequently, an Ag film or Al film is formed as a back electrode 17 onthe second transparent electrode 46.

A tandem photovoltaic device produced in this manner generateselectricity by photovoltaic conversion from incident light such assunlight that enters the amorphous silicon photovoltaic layer 30 and themicrocrystalline silicon layer 40 with the PIN structures describedabove via the transparent electrically insulating substrate 11.

In the production of the photovoltaic device, the amorphous siliconphotovoltaic layer 30 was formed with a PIN structure by sequentialformation of the p-type silicon layer 33, the i-type silicon layer 34,and the n-type silicon layer 35 on top of the first transparentelectrode 42, but the photovoltaic layer 30 may also be formed with aNIP structure by sequential formation of an n-type silicon layer i-typesilicon layer, and p-type silicon layer.

Furthermore, the microcrystalline silicon photovoltaic layer 40 wasformed with a PIN structure by sequential formation of the p-typesilicon layer 43, the i-type silicon layer 44, and the n-type siliconlayer 45 from the side of the first transparent electrode 42, but thephotovoltaic layer 40 may also be formed with a NIP structure bysequential formation of an n-type silicon layer i-type silicon layer,and p-type silicon layer.

Furthermore, in this embodiment, the ZnO layer in which the quantity ofadded Ga relative to Zn was restricted to not more than 5 atomic % wasused for the second transparent electrode 46, but the present inventionis not limited to this case, and the above ZnO layer may also be usedfor the first transparent electrode 32.

However, because the transparent electrode also has the effect ofincreasing the reflectance, the Ga-doped ZnO layer of the presentinvention is preferably employed as the second transparent electrode 46positioned adjacent to the back electrode 17.

According to this embodiment, Ga-doped ZnO was employed as the secondtransparent electrode 46, the quantity of added Ga relative to Zn wasrestricted to not more than 5 atomic %, the quantity of oxygen added tothe sputtering gas during formation of the Ga-doped ZnO layer was set toa value of not less than 0.1% by volume and not more than 5% by volumerelative to the combined volume of argon and oxygen within thesputtering gas, and the quantity of added Ga was reduced as far aspossible within a range that enabled the desired photovoltaic conversionefficiency to be maintained, with some allowance for an increase in theresistivity of the second transparent electrode 46, and as a result,reductions in the transmittance caused by the Ga addition weresuppressed, enabling the production of a second transparent electrode 46with high transmittance over a wide range of wavelengths. Accordingly,because there is no longer any necessity to add oxygen during theformation of the ZnO layer in order to improve the transmittance, damageto the transparent electrode caused by oxygen can be reduced, whichimproves the controllability and yield during film formation.

Because a high transmittance is achieved in this manner, more intenselight can be supplied to the photovoltaic layer, thereby increasing theshort-circuit current density, and as a result, increasing thephotovoltaic conversion efficiency.

Furthermore, suppressing the quantity of added Ga enhances the interfaceproperties with the p-type and n-type silicon layers, enabling a highopen-circuit voltage, a favorable short-circuit current density and afavorable fill factor to be achieved, and as a result, the photovoltaicconversion efficiency improves.

In the above first through third embodiments, the descriptions focusedon applications of the present invention to superstrate photovoltaicdevices, but the present invention is not limited to this configuration,and may also be applied to substrate photovoltaic devices. In suchcases, a Ga-doped ZnO layer of the present invention can be used for thetransparent electrode on the substrate side, the transparent electrodeon the light incident side, or for both of these transparent electrodes.

EXAMPLES

Examples of the present invention are described below.

First Test Example

In a first test example, photovoltaic devices of examples 1 to 4 wereprepared with the same layer configuration as that of the firstembodiment. Specifically, single photovoltaic devices including a singleamorphous silicon photovoltaic layer 10 in which incident light entersfrom the side of the transparent electrically insulating substrate 11were prepared as shown in FIG. 1.

The first transparent electrode 12 was SnO₂. The quantity of added Garelative to Zn within the second transparent electrode 16, and thequantity of oxygen added to the sputtering gas used during formation ofthe Ga-doped ZnO layer, relative to the combined volume of argon andoxygen within the sputtering gas, were set to the values shown in Table1.

The film thickness of the second transparent electrode 16 was 80 nm.

In each case, the transmittance of the second transparent electrode 16was 95% or greater within the wavelength region of 550 nm or greater.

As a comparative example 1, a photovoltaic device was prepared in thesame manner as the example 1 through example 4, with the exceptions ofsetting the quantity of added Ga relative to Zn within the ZnO of thesecond transparent electrode 16 to 6 atomic %, and setting the quantityof oxygen added to the sputtering gas used during formation of theGa-doped ZnO layer, relative to the combined volume of argon and oxygenwithin the sputtering gas, to 0% by volume.

Second Test Example

In a second test example, photovoltaic devices of examples 5 to 8 wereprepared with the same layer configuration as that of the secondembodiment. Specifically, single photovoltaic devices including a singlemicrocrystalline silicon photovoltaic layer 20 in which incident lightenters from the side of the transparent electrically insulatingsubstrate 11 were prepared as shown in FIG. 2.

The first transparent electrode 22 was SnO₂. The quantity of added Garelative to Zn within the second transparent electrode 26, and thequantity of oxygen added to the sputtering gas used during formation ofthe Ga-doped ZnO layer, relative to the combined volume of argon andoxygen within the sputtering gas, were set to the values shown in Table2.

The film thickness of the second transparent electrode 26 was 80 nm.

In each case, the transmittance of the second transparent electrode 26was 95% or greater within the wavelength region of 550 nm or greater.

As a comparative example 2, a photovoltaic device was prepared in thesame manner as the example 5 through example 8, with the exceptions ofsetting the quantity of added Ga relative to Zn within the ZnO of thesecond transparent electrode 26 to 6 atomic %, and setting the quantityof oxygen added to the sputtering gas used during formation of theGa-doped ZnO layer, relative to the combined volume of argon and oxygenwithin the sputtering gas, to 0% by volume.

Third Test Example

In a third test example, photovoltaic devices of examples 9 to 12 wereprepared with the same layer configuration as that of the thirdembodiment. Specifically, tandem photovoltaic devices including a singleamorphous silicon photovoltaic layer 30 and a single microcrystallinesilicon photovoltaic layer 40, in which incident light enters from theside of the transparent electrically insulating substrate 11, wereprepared as shown in FIG. 3.

The first transparent electrode 32 was SnO₂. The quantity of added Garelative to Zn within the second transparent electrode 46, and thequantity of oxygen added to the sputtering gas used during formation ofthe Ga-doped ZnO layer, relative to the combined volume of argon andoxygen within the sputtering gas, were set to the values shown in Table3.

The film thickness of the second transparent electrode 46 was 80 nm.

In each case, the transmittance of the second transparent electrode 46was 95% or greater within the wavelength region of 550 nm or greater.

As a comparative example 3, a photovoltaic device was prepared in thesame manner as the example 9 through example 12, with the exceptions ofsetting the quantity of added Ga relative to Zn within the ZnO of thesecond transparent electrode 46 to 6 atomic %, and setting the quantityof oxygen added to the sputtering gas used during formation of theGa-doped ZnO layer, relative to the combined volume of argon and oxygenwithin the sputtering gas, to 0% by volume.

The electric power generation performance of the photovoltaic devices ofthe above Examples 1 to 12 and the comparative examples 1 to 3corresponding with those examples was evaluated by irradiating thetransparent electrically insulating substrate 11 of each photovoltaicdevice with simulated sunlight (spectral type: AM 1.5; irradiationintensity: 100 mW/m²; irradiation temperature: 25° C.). The results areshown in Table 1 to Table 3.

TABLE 1 First Test Example (a-Si single) Comparative Exam- Exam- example1 Example 1 Example 2 ple 3 ple 4 Ga (at. %) 6 1 1.5 5 0.05 O₂ (vol. %)0 1 0.1 2 5 Jsc (short- 1 1.09 1.1 1.1 1.09 circuit current density) Voc(open- 1 1 1 1 1 circuit voltage) FF (fill factor) 1 1 1 1 1 Eff. 1 1.031.04 1.04 1.02 (conversion efficiency)

TABLE 2 Second Test Example (microcrystalline Si single) ComparativeExam- Exam- example 2 Example 5 Example 6 ple 7 ple 8 Ga (at. %) 6 1 20.2 4 O₂ (vol. %) 0 1.7 0.1 3 5 Jsc (short- 1 1.1 1.1 1.1 1.11 circuitcurrent density) Voc (open- 1 1 1 1 1 circuit voltage) FF (fill factor)1 1.01 1 1.01 1 Eff. 1 1.08 1.07 1.08 1.08 (conversion efficiency)

TABLE 3 Third Test Example (tandem) Comparative Exam- Example ExampleExample example 3 ple 9 10 11 12 Ga (at. %) 6 1 1 4 0.1 O₂ (vol. %) 01.5 3 0.5 4 Jsc (short- 1 1.1 1.1 1.09 1.09 circuit current density) Voc(open- 1 1 1 1 1 circuit voltage) FF (fill 1 1.01 1.01 1 1.01 factor)Eff. 1 1.05 1.05 1.03 1.03 (conversion efficiency)

Table 1 shows the Jsc (short-circuit current density), Voc (open-circuitvoltage), FF (fill factor) and Eff. (conversion efficiency) for eachexample and the comparative example. In each of the test examples, thevalues for each example are shown as relative values wherein themeasured value of the respective comparative example was set to 1.

As is evident from Table 1 to Table 3, reducing the quantity of added Garelative to Zn within the ZnO of the transparent electrode comprisingthe Ga-doped ZnO layer, and adding oxygen to the sputtering gas usedduring formation of the Ga-doped ZnO layer enables high transmittance tobe achieved over a wide range of wavelengths, meaning more intense lightcan be supplied to the photovoltaic layer, thereby increasing the Jsc(short-circuit current density).

Because the Jsc (short-circuit current density) and the FF (fill factor)are improved in this manner, it is evident that reducing the quantity ofadded Ga within the Ga-doped ZnO transparent electrode yields anincrease in the conversion efficiency.

In particular, the microcrystalline silicon single photovoltaic devicesof the Test Example 2 exhibit significant improvement in the Eff.(conversion efficiency). It is thought that the reason for thisimprovement is that the decrease in the quantity of Ga causes animprovement in the properties at the interface between the n-layer andthe Ga-doped ZnO layer.

1. A photovoltaic device comprising at least a first transparentelectrode, a first photovoltaic layer containing mainly amorphoussilicon or microcrystalline silicon, and a second transparent electrodelaminated sequentially on top of an electrically insulating substrate,wherein at least one of the first transparent electrode and the secondtransparent electrode is either a ZnO layer containing no Ga, or aGa-doped ZnO layer in which a quantity of added Ga is not more than 5atomic % relative to Zn within the ZnO layer, and the ZnO layer isformed by a sputtering method using a rare gas containing added oxygenas a sputtering gas, wherein a quantity of oxygen added to thesputtering gas is not less than 0.1% by volume and not more than 5% byvolume relative to a combined volume of the oxygen and the rare gas. 2.A photovoltaic device comprising at least a first transparent electrode,a first photovoltaic layer containing mainly amorphous silicon ormicrocrystalline silicon, and a second transparent electrode laminatedsequentially on top of an electrically insulating substrate, wherein atleast one of the first transparent electrode and the second transparentelectrode is either a ZnO layer containing no Ga, or a Ga-doped ZnOlayer in which a quantity of added Ga is not more than 5 atomic %relative to Zn within the ZnO layer, and the ZnO layer is formed by aphysical vapor deposition method using a rare gas containing addedoxygen as a reactive gas, wherein a quantity of oxygen added to thereactive gas is not less than 0.1% by volume and not more than 5% byvolume relative to a combined volume of the oxygen and the rare gas. 3.A photovoltaic device according to claim 1, wherein the firstphotovoltaic layer contains mainly microcrystalline silicon, and asecond photovoltaic layer containing mainly amorphous silicon isprovided between the first photovoltaic layer and the first transparentelectrode.
 4. A photovoltaic device according to claim 2, wherein thefirst photovoltaic layer contains mainly microcrystalline silicon, and asecond photovoltaic layer containing mainly amorphous silicon isprovided between the first photovoltaic layer and the first transparentelectrode.
 5. A process for producing a photovoltaic device comprisingat least a first transparent electrode, a first photovoltaic layercontaining mainly amorphous silicon or microcrystalline silicon, and asecond transparent electrode laminated sequentially on top of anelectrically insulating substrate, wherein the process comprises a stepof forming at least one of the first transparent electrode and thesecond transparent electrode by a sputtering method that uses a targetcontaining mainly ZnO, and a rare gas containing added oxygen as asputtering gas, the target is either a target containing no Ga, or aGa-doped target in which a quantity of added Ga is not more than 5atomic % relative to Zn within the ZnO, and a quantity of oxygen addedto the sputtering gas is not less than 0.1% by volume and not more than5% by volume relative to a combined volume of the oxygen and the raregas.
 6. A process for producing a photovoltaic device comprising atleast a first transparent electrode, a first photovoltaic layercontaining mainly amorphous silicon or microcrystalline silicon, and asecond transparent electrode laminated sequentially on top of anelectrically insulating substrate, wherein the process comprises a stepof forming at least one of the first transparent electrode and thesecond transparent electrode by a physical vapor deposition method thatuses a vapor deposition material containing mainly ZnO, and a rare gascontaining added oxygen as a reactive gas, the vapor deposition materialis either a vapor deposition material containing no Ga, or a Ga-dopedvapor deposition material in which a quantity of added Ga is not morethan 5 atomic % relative to Zn within the ZnO, and a quantity of oxygenadded to the reactive gas is not less than 0.5% by volume and not morethan 5% by volume relative to a combined volume of the oxygen and therare gas.
 7. A process for producing a photovoltaic device according toclaim 5, wherein the first photovoltaic layer contains mainlymicrocrystalline silicon, and the process comprises a step of forming asecond photovoltaic layer containing mainly amorphous silicon betweenthe first photovoltaic layer and the first transparent electrode.
 8. Aprocess for producing a photovoltaic device according to claim 6,wherein the first photovoltaic layer contains mainly microcrystallinesilicon, and the process comprises a step of forming a secondphotovoltaic layer containing mainly amorphous silicon between the firstphotovoltaic layer and the first transparent electrode.